Momentum

Which is more effective at demolishing a brick wall, a 1 kg iron ball moving at 50 m s^–1 or a 1000 kg iron ball moving at 1 m s^–1? Both the mass and the velocity need to be taken into account to answer this question.

Newton realised that what happens to a moving object involved in a collision depends on two factors:

• the mass of the object
• the velocity of the object.

He used the concept of momentum to explain the results of collisions between objects.

A collision between two objects need not involve physical contact. Imagine two positive charges approaching each other; the repulsive forces could cause them to reverse in direction without actually touching.

momentum = mass × velocity
p = mv
Momentum is a vector quantity, the direction being the same as that of the velocity. It is measured in N s or kg m s^–1; these units are equivalent.

Everything that moves has momentum and exerts a force on anything that it interacts with. This applies as much to the light that is hitting you at the moment as it does to a collision between two vehicles.

A ten-pin bowling ball has a mass of several kilogrammes, so even at low speeds it has much more momentum than a tennis ball (mass 0.06 kg) travelling as fast as you can throw it.

No matter how hard you throw a tennis ball at a garden wall you will not knock the wall down; nor will you be very successful if you use a tennis ball to play tenpin bowling!